(1) Field of the Invention
The present invention relates to a carrier for a developer, which is used in the electrophotographic process of the like. More particularly, the present invention relates to a carrier for a developer, which has highly improved flowability and durability.
(2) Description of the Related Art
In the electrophotographic process using a two-component type developer, a toner composed of colored resin particles comprising a colorant dispersed in a binder resin is mixed with a carrier composed of iron powder or ferrite, this two-component type developer is supplied onto a developing sleeve, in which magnets are disposed, to form a magnetic brush of this developer composition, and this magnetic brush is brought into frictional contact with a photosensitive layer of a photoconductive substance having an electrostatic latent image to form a toner image on the photosensitive layer. The toner gets a desired frictional charge by the friction with the magnetic carrier, and the toner on the magnetic brush is transported to the electrostatic latent image on the photosensitive layer by an electrostatic force or the like to adhere to the photosensitive layer and effect the development of the electrostatic latent image. The toner image formed by the development on the photosensitive layer is transferred onto a transfer material such as a transfer sheet and is fixed onto the transfer material by heat or pressure to form an image.
Since the toner in the developing device is consumed for the formation of images, in order to perform the formation of images repeatedly, it is necessary that a fresh toner should be supplied in an amount corresponding to the consumption into the developing device and should be promptly charged by stirring and friction with the magnetic carrier. However, while the developing operation is repeated, a toner film is formed on the surface because of deterioration called "spending phenomenon". Moreover, since the magnetic carrier is hygroscopic, good control of the charge becomes impossible. Accordingly, for overcoming this advantage, there has been adopted a method in which the magnetic carrier is coated with a resin or silicone oil to prevent the spending phenomenon and impart a moisture resistance to the carrier. In a carrier having this coating layer, the above-mentioned disadvantage can be overcome to some extent, and furthermore, another merit is attained in that the electric resistance can be freely adjusted. Therefore, carriers of this type are now used in large quantities, and many carriers having a resin coating layer in which a silicone oil is incorporated have recently been proposed.
Recently, in an image-forming apparatus such as a copying machine, the speed is increased and images are formed at a speed of 50 to 70 sheets (A-4 size) per minute. Moreover, the frequency of use of the copying machine is recently increased and hence, stirring of the developer is carried out at a high speed frequently. Accordingly, the improvement of the durability of the developer, especially the carrier, is an important technical problem.
The carrier having silicone oil incorporated in the above-mentioned coating layer is improved to some extent over the uncoated carrier in the moisture resistance and the prevention of the spending phenomenon. However, under severe copying operation conditions where stirring is carried out at a high speed or continuously for a long time, the surface state of the coating layer is degraded presumably because the fixation between the silicone oil and the carrier core material or between the silicone oil and the coating resin is insufficient, and the flowability of the developer is degraded by the stickiness of the silicone oil and rising of the charging is insufficient, with the result that charge quantity is often changed and fogging is caused or the image density is reduced. This disadvantage becomes conspicuous as the copying operation is continued.